01. StreamEvent type theorem

StreamEvent type theorem

Default event table

Event type	Event name	Input data	Output/chunk data
Chat Model	on_chat_model_start	{"messages": [[SystemMessage, HumanMessage]]}	-
Chat Model	on_chat_model_stream	-	AIMessageChunk (content="hello")
Chat Model	on_chat_model_end	{"messages": [[SystemMessage, HumanMessage]]}	AIMessageChunk (content="hello world")
LLM	on_llm_start	{'input':'hello'}	-
LLM	on_llm_stream	-	'Hello'
LLM	on_llm_end	-	'Hello human!'
Chain	on_chain_start	-	-
Chain	on_chain_stream	-	"hello world!, goodbye world!"
Chain	on_chain_end	[Document (...)]	"hello world!, goodbye world!"
Tool	on_tool_start	{"x": 1, "y": "2"}	-
Tool	on_tool_end	-	{"x": 1, "y": "2"}
Retriever	on_retriever_start	{"query": "hello"}	-
Retriever	on_retriever_end	{"query": "hello"}	[Document (...), ..]
Prompt	on_prompt_start	{"question": "hello"}	-
Prompt	on_prompt_end	{"question": "hello"}	ChatPromptValue (messages: [SystemMessage, ...])

StreamEvent structure

property	type	Explanation	Remarks
event	str	Event name	on_[runnable_type]_(start|stream|end) format
name	str	Runnable name	-
run_id	str	Execution unique ID	Newly created every run
parent_ids	List[str]	Parent Runnable IDs	only available on v2 API
tags	Optional[List[str]]	Runnable tag	Selector
metadata	Optional[Dict[str, Any]]	Runnable metadata	Selector
data	Dict[str, Any]	Event data	-

Custom event structure

property	type	Explanation
name	str	Custom event name
data	Any	Event-related data (JSON serialization recommended)